1. Field of the Invention
The present invention relates to a semiconductor device including a circuit including a semiconductor element and a manufacturing method thereof. In addition, the present invention relates to an electronic appliance to which a light emitting display device including an organic light emitting element or an electro-optic device typified by a liquid crystal display panel is mounted as a part component. Note that in this specification, a semiconductor device refers to a device in general which functions by utilizing semiconductor characteristics. Electro-optic devices, semiconductor circuits, and electronic appliances are all semiconductor devices.
2. Description of the Related Art
Conventionally, a structure is widely known where thin film transistors (also referred to as TFTs) are integrated to be utilized, in the case where a circuit in an active matrix liquid crystal display device, an image sensor, or the like is formed over a glass substrate. In this case, a wiring over a substrate is generally formed by forming a first wire which is a lower wire, forming an interlayer insulator, and then, forming a second wire which is an upper wire. If necessary, a third layer and a fourth layer of a wiring may be formed.
In addition, a TFT manufacturing process is known where a photo mask (hereinafter, referred to as a light exposure mask) or a reticle each of which is provided with a diffraction grating pattern or with an auxiliary pattern formed of a semi-translucent film having a light intensity reducing function is applied to a photolithography step for forming a gate electrode (see Patent Document 1).    Patent Document 1 Japanese Published Patent Application No. 2000-151523
A problem of a semiconductor device including a semiconductor element is a breaking (that is, a breakage caused at a step corner) of an upper wire in a portion where a lower wire and the upper wire formed thereover intersect with each other (an overlapping portion). The problem is caused because an edge of the lower wire is steep and an interlayer insulating film formed thereover does not sufficiently cover the lower wire.
To prevent such a breaking, it is required to thicken the upper wire. For example, the upper wire is desirably about twice as thick as a gate wire which is the lower wire. However, this means that unevenness of an integrated circuit is further increased. Besides, when a wire is required to be formed over the integrated circuit, breaking of the wire due to the thickness of the upper wire has to be taken into consideration. In addition, when forming a circuit in which unevenness of an integrated circuit is unfavorable like a liquid crystal display device, it is practically impossible to increase the thickness of the upper wire.